Relay device and communication system

ABSTRACT

A relay device requests a status notification of another relay device to a server device. When the relay device is notified that the other relay device is under a logged-out status, the relay device stands by. When the relay device is notified that the other relay device has logged in, the relay device transmits a connection request to the server device. The server device relays the connection request to the other relay device. A response is transmitted from the other relay device to the server device, and further relayed to the relay device. Accordingly, a TCP connection is established between the relay device and the other relay device. A communication device connected to a different network can transmit and receive data to and from the relay device via the other relay device.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation application under 37 C.F.R. §1.53 (b)of U.S. application Ser. No. 11/723,466, filed Mar. 20, 2007, which isincorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to technology for relaying datatransmitted and received between communication devices.

2. Description of the Related Art

By using technology such as a Virtual Private Network (VPN) andtunneling, data transmitted from a terminal located inside a privatenetwork can be relayed to a terminal located in another private networkvia the Internet. For example, by establishing a connection between aLocal Area Network (LAN) of a headquarter office and a LAN of a branchoffice via the VPN, data can be relayed between terminals connected todifferent LANs via the Internet.

According to known technology, data is relayed between local systemsconnected via the Internet. According to this technology, each terminallocated within a local system accesses a relay server on the Internet toestablish a Transmission Control Protocol (TCP) connection, and uses theestablished TCP connection to transmit and receive data between thelocal systems.

By using the technology of the VPN and the tunneling, data can betransmitted and received between different LANs connected via theInternet. However, such technology provides a system which isconstructed by a fixed setting. That is, a relay device provided on theInternet between the LANs carries out a fixed connection setting betweenthe LANs. Therefore, there is no problem when a VPN connection ispreferably established constantly between a headquarter office and abranch office of a company, for example. However, the above-describedtechnology cannot support a case in which a connection is to beestablished dynamically between different private networks.

The technology such as the VPN is basically technology for establishinga connection between networks, for example, between a LAN and a LAN, andis not technology for establishing a connection between individualterminals. Therefore, the technology such as the VPN is technology forenabling communication to be carried out between networks in which theVPN is developed as an infrastructure, and is not technology forsupporting a case in which an individual personally demands acommunication with a specific network.

Meanwhile, when constructing a system in which a connection isestablished between networks in response to a request from individualterminals, performance of a relay device may decrease due to a fact thatresources may become insufficient when a plurality of connections aregenerated according to the request from terminals.

SUMMARY OF THE INVENTION

In order to overcome the problems described above, preferred embodimentsof the present invention provide a system in which relaying of databetween networks connected via the Internet can be set dynamically.

Preferred embodiments of the present invention also provide a system inwhich relaying of data between networks connected via the Internet canbe set dynamically according to a request of each terminal.

According to an aspect of the present invention, a relay device cancarry out communication with a first communication device, and includesa relay unit and a holding unit. The relay unit relays data receivedfrom the first communication device to another relay device. The holdingunit inquires a status of the other relay device to a server device.When the other relay device is capable of accepting a connection, theholding unit carries out a connection request to the other relay device,dynamically establishes a relay connection with the other relay device,and holds the established relay connection. The other relay device cancarry out communication with a second communication device. The relaydevice relays the data received from the first communication device tothe other relay device to further relay the data to the secondcommunication device.

The holding unit also establishes relay connections with a plurality ofrelay devices, and the relay unit uses the plurality of the relayconnections to relay the data to the plurality of the relay devices.

The holding unit can separately disconnect the relay connections heldwith the plurality of the relay devices.

According to another aspect of the present invention, a communicationsystem for relaying data between terminals includes a first relay devicethat can carry out communication with a first communication device, aserver device, and a second relay device that can carryout communicationwith a second communication device. The first relay device includes aholding unit. When the holding unit inquires a status of the secondrelay device to the server device and the second relay device is capableof accepting a connection, the holding unit carries out a connectionrequest to the second relay device, dynamically establishes a relayconnection with the second relay device, and holds the established relayconnection. The first relay device relays data received from the firstcommunication device to the second relay device, and the second relaydevice further relays the data to the second communication device.

The first communication device and the second communication device arerespectively provided in different private networks. Accordingly, aconnection request for a TCP connection cannot be carried out to thefirst communication device and the second communication device from anexternal network.

According to another aspect of the present invention, a relay device cancarry out communication with a first communication device. The relaydevice includes an accepting unit and a holding unit. The accepting unitaccepts a relay transmission instruction designating another relaydevice from the first communication device. The holding unit inquires astatus of the other relay device to the server device. When the otherrelay device is capable of accepting a connection, the holding unitcarries out a connection request to the other relay device, dynamicallyestablishes a relay connection with the other relay device, and holdsthe established relay connection. The other relay device can carry outcommunication with a second communication device. The relay devicerelays data received from the first communication device to the otherrelay device, and the other relay device further relays the data to thesecond communication device.

When the other relay device is incapable of accepting a connection, theholding unit notifies such a connection impossible state to the firstcommunication device. When the holding unit receives a notification fromthe server device that the other relay device has become capable ofaccepting a connection, the holding unit notifies a connection possiblestate to the first communication device.

Only data, which has the first communication device as a transmitter ora transmission destination, is relayed over the relay connection held bythe holding unit.

According to another aspect of the present invention, a communicationsystem for relaying data between terminals includes a first relay devicethat can carry out communication with a first communication device, aserver device, and a second relay device that can carry outcommunication with a second communication device. The first relay deviceincludes an accepting unit and a holding unit. The accepting unitaccepts a relay instruction of data designating the second relay devicefrom the first communication device. The holding unit inquires a statusof the second relay device to the server device. When the second relaydevice is capable of accepting a connection, the holding unit carriesout a connection request to the second relay device, dynamicallyestablishes a relay connection with the second relay device, and holdsthe established relay connection. The first relay device relays datareceived from the first communication device to the second relay deviceso that the second relay device can further relay the data to the secondcommunication device.

The first communication device and the second communication device arerespectively provided in different private networks. A connectionrequest for a TCP connection cannot be carried out to the firstcommunication device and the second communication device from anexternal network.

According to another aspect of the present invention a relay device cancarry out communication with a first communication device. The relaydevice includes an accepting unit and a holding unit. The accepting unitaccepts a relay transmission instruction designating another relaydevice from the first communication device. When a relay connection hasnot been established yet with the other relay device, the holding unitinquires a status of the other relay device to the server device. Whenthe other relay device is capable of accepting a connection, the holdingunit dynamically establishes a relay connection with the other relaydevice, and holds the established relay connection. When a relayconnection is already established with the other relay device, theholding unit sets such relay connection to be shared with the firstcommunication device. The other relay device can carry out communicationwith the second communication device. The relay device relays datareceived from the first communication device to the other relay device,and the other relay device further relays the data to the secondcommunication device.

When receiving a notification from the first communication device toterminate the relay transmission to the other relay device, if the othercommunication device is carrying out a relay transmission to the otherrelay device, the relay connection established between the relay deviceand the other relay device is maintained. When none of the communicationdevices is carrying out a relay transmission to the other relay device,the holding unit disconnects the relay connection held with the otherrelay device.

According to another aspect of the present invention, a communicationsystem for relaying data between terminals includes a first relay devicethat can carry out communication with a first communication device, aserver device, and a second relay device that can carry outcommunication with the second communication device. The first relaydevice includes an accepting unit and a holding unit. The accepting unitaccepts a relay transmission instruction of data designating the secondrelay device from the first communication device. When a relayconnection is yet to be established with the second relay device, theholding unit inquires a status of the second relay device to the serverdevice. When the second relay device is capable of accepting aconnection, the holding unit dynamically establishes a relay connectionwith the second relay device and holds the established relay connection.When a relay connection is already established with the second relaydevice, the holding unit sets such relay connection to be shared withthe first communication device. The first relay device relays datareceived from the first communication device to the second relay device,and the second relay device further relays the data to the secondcommunication device.

The first communication device and the second communication device arerespectively provided in different private networks. A connectionrequest for a TCP connection cannot be carried out to the firstcommunication device and the second communication device from anexternal network.

The relay device of the present invention inquires a status of anotherrelay device to the server device. When the other relay device iscapable of accepting a connection, the relay device carries out aconnection request, and establishes and holds a relay connection withthe other relay device. Therefore, a communication device, which cancarry out communication with the relay device, can carry outcommunication via the private network with a communication device, whichcan carry out communication with the other relay device.

The relay device of the present invention can establish a relayconnection with a plurality of relay devices, and hold the establishedrelay connections. Therefore, when communication devices to whichcommunication is to be carried out belong to different networks, aplurality of connections can be generated independently.

The relay device of the present invention can independently disconnecteach of the relay connections being held with a plurality of the relaydevices. Therefore, the relay device of the present invention canmaintain only the connections that are necessary for communication, andresources can be utilized efficiently.

The relay device of the present invention accepts a relay transmissioninstruction designating the other relay device from the firstcommunication device, and inquires a status of the other relay device tothe server device. When the other relay device is capable of accepting aconnection, the relay device dynamically establishes a relay connectionwith the other relay device and holds the established relay connection.Therefore, by carrying out a request designating a relay destination tothe relay device of the present invention, a communication device, whichcan carry out communication with the relay device of the presentinvention, can carryout communication with a communication device, whichcan carry out communication with the other relay device. Since aconnection is established between the relay devices according to arequest from the communication device, a network of a communicationdestination can be selected and a connection can be established moredynamically.

When receiving a notification from the server device that the otherrelay device has become capable of accepting a connection, the relaydevice notifies a connection possible state to the first communicationdevice. Accordingly, at a point of time when a preparation is completedat the relay destination, a relay transmission can be startedimmediately.

Only the data having the first communication device as a transmitter ora transmission destination is relayed over the relay connection held bythe holding unit. Therefore, a terminal, which has carried out aconnection request to the relay device, can exclusively use theestablished connection.

The relay device of the present invention accepts a relay transmissioninstruction designating another relay device from the firstcommunication device. When a relay connection is already establishedbetween the relay device of the present invention and the other relaydevice, the relay device of the present invention sets the firstcommunication device to share the already established relay connection.Therefore, by carrying out a request designating a relay destination tothe relay device of the present invention, a communication device, whichcan carry out communication with the relay device of the presentinvention, can carryout communication with a communication device, whichcan carry out communication with the other relay device. When a relayconnection is already established between the relay device of thepresent invention and the designated relay device, the alreadyestablished relay connection is shared. Therefore, processing load forestablishing a connection can be reduced, and resources can be saved.

When receiving a notification from the first communication device toterminate the relay transmission to the other relay device, if the othercommunication device is carrying out a relay transmission to the otherrelay device, the relay connection established with the other relaydevice is maintained. Accordingly, an end of communication of onecommunication device does not influence a communication of anothercommunication device. A communication device sharing the relayconnection can use the relay connection until the communication of thecommunication device itself is completed.

Other features, elements, processes, steps, characteristics andadvantages of the present invention will become more apparent from thefollowing detailed description of preferred embodiments of the presentinvention with reference to the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a configuration of a communication system accordingto a first preferred embodiment of the present invention.

FIG. 2 illustrates a registration example of a relay device database.

FIG. 3 is a flowchart illustrating a processing carried out in thecommunication system according to the first preferred embodiment of thepresent invention.

FIG. 4 is a flowchart illustrating a processing carried out in thecommunication system focusing on a relay device.

FIG. 5 is a flowchart illustrating a processing carried out in thecommunication system focusing on a relay device.

FIG. 6 illustrates an imaginary state in which a plurality of relayconnections are established.

FIG. 7 illustrates a configuration of a communication system accordingto a second preferred embodiment of the present invention.

FIG. 8 illustrates a registration example of a relay connectiondatabase.

FIG. 9 is a flowchart illustrating a processing carried out in thecommunication system according to the second preferred embodiment of thepresent invention.

FIG. 10 is a flowchart illustrating a processing carried out in thecommunication system focusing on a relay device.

FIG. 11 is a flowchart illustrating a processing carried out in thecommunication system focusing on a relay device.

FIG. 12 illustrates a registration example of a relay connectiondatabase.

FIG. 13 is a flowchart illustrating a processing carried out in acommunication system according to a third preferred embodiment of thepresent invention.

FIG. 14 is a flowchart illustrating a processing carried out in thecommunication system focusing on a relay device.

FIG. 15 is a flowchart illustrating a processing carried out in thecommunication system focusing on a relay device.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS First Preferred Embodiment

Preferred embodiments of the present invention will be described withreference to the accompanying drawings. FIG. 1 illustrates aconfiguration of a communication system according to a first preferredembodiment. In the communication system of the first preferredembodiment, two Local Area Networks (LANs) 10 and 20 are connected via aWide Area Network (WAN) 30. For example, the LANs 10 and 20 are anin-house LAN of a company, and the WAN 30 is a public network such asthe Internet. That is, two different private LANs 10 and 20 areconnected via a public network such as the Internet.

As illustrated in FIG. 1, two communication devices 11 and 12 and arelay device 15 are connected in the LAN 10. The LAN 10 is connected tothe WAN 30 via a gateway (GW) 16. Two communication devices 21 and 22and a relay device 25 are connected in the LAN 20. The LAN 20 isconnected to the WAN 30 via a gateway 26.

The communication devices 11 and 12 are terminals such as personalcomputers, and include a network function. Specifically, each of thecommunication devices 11 and 12 includes a Transmission ControlProtocol/Internet Protocol (TCP/IP), and uses the TCP/IP to carry outcommunication with the relay device 15 and other computers or the likeconnected to the LAN 10. In the same manner, each of the communicationdevices 21 and 22 includes a TCP/IP, and uses the TCP/IP to carry outcommunication with the relay device 25 and other computers or the likeconnected to the LAN 20. The relay devices 15 and 25 and the gateways 16and 26 can also carry out communication by the TCP/IP.

A server device 35 is connected to the WAN 30. As described above, theWAN 30 is a public network such as the Internet. Therefore, a terminalconnected to the LAN 10, the LAN 20, or the WAN 30 can establish a TCPconnection by designating a global IP address of the server device 35.

A private IP address is assigned to the communication devices 11 and 12and the relay device 15 connected to the LAN 10 and the communicationdevices 21 and 22 and the relay device 25 connected to the LAN 20. Thegateways 16 and 26 function as a firewall, and a connection cannot beestablished directly from an external network by designating a terminallocated inside of the network. Therefore, a terminal or the likeconnected to the WAN 30 has restrictions when carrying out a TCPconnection request to the relay device 15 or 25. In the first preferredembodiment, since the server device 35 functions as a Session InitiationProtocol (SIP) server (a proxy server and a registered server of theSIP) as to be described hereinafter, the gateways 16 and 26 are set suchthat communication designating a specific port corresponding to therelay servers 15 and 25 can be carried out to the gateways 16 and 26,respectively.

FIG. 2 illustrates a registration example of a relay device database 351managed by the server device 35. In the example illustrated in FIG. 2,three relay devices A, B, and C are registered. The relay devices A, B,and C are device names that are assigned to the relay device 15 or therelay device 25, for example. Flags of a Uniform Resource Locator (URL),an IP address, and a login status are set for each of the relay devicesA, B, and C.

An IP address of each of the relay devices registered in the relaydevice database 351 is not an IP address in a private network, but is aglobal IP address converted by a gateway by a function such as a NetworkAddress Translation (NAT) or an IP masquerade. For example, the relaydevice 15 is assigned with a private address in the LAN 10. When therelay device 15 accesses the server device 35, the IP address assignedto the relay device 15 is converted into a global IP address by thegateway 16.

Each of the relay devices A through C establishes a connection with theserver device 35, and logs into the server device 35. A “status” fieldof the relay device database 351 stores information indicating whethereach of the relay devices A through C is currently under a logged-instatus or a logged-out status with respect to the server device 35. Wheneach of the relay devices is under a logged-in status, the server device35 determines that a TCP connection is prepared to be establishedbetween the logged-in relay devices. In other words, a status in whichthe relay device is logged in the server device 35 indicates a status inwhich the relay device is capable of accepting a TCP connection requesttransmitted from another terminal.

With reference to FIG. 3 through FIG. 5, a description will be made of aflow of a communication processing in the above-described communicationsystem. FIG. 3 illustrates a flow of the processing of the entirecommunication system including the relay devices 15 and 25 and theserver device 35. Further, in the following description, a SessionInitiation Protocol (SIP) is used for communication between the relaydevice 15 and the server device 35, and between the relay device 25 andthe server device 35. However, other protocols may also be used.

Each of the relay devices 15 and 25 transmits a “REGISTER requestmessage” of the SIP to the server device 35 at initialization orperiodically to notify position information (for example, an IP address,and/or a port number) of the relay device to the server device 35. Theserver device 35 manages the relay device database 351 illustrated inFIG. 2 according to the notified position information. The server device35 can carry out communication with each of the relay devices over thegateway according to the position information. In FIG. 3, under aninitial state, the relay device 15 and the relay device 25 are logged inthe server device 35. First, the relay device 25 notifies logout statusinformation to the server device 35 (step S101), and the server device35 responds to the notification (step S102). The server device 35executes a logout processing of the relay device 25, and updates the“status” field of the relay device 25 in the relay device database 351to “logout”.

Under this state, when the relay device 15 carries out a notificationrequest of the status information to the server device 35 (step S103),the server device 35 carries out a response (step S104), and notifiesthat the relay device 25 is logged-out (step S105). The relay device 15responds to this notification (step S106).

Next, the relay device 25 notifies login status information to theserver device 35 (step S107), and the server device 35 responds to thenotification (step S108). The server device 35 executes the loginprocessing of the relay device 25, and updates the “status” field of therelay device database 351 to “login”. Furthermore, the server device 35notifies the relay device 15 that the relay device 25 has logged in(step S109). The relay device 15 responds to this notification (stepS110).

When the relay device 15 receives a notification that the relay device25 has logged in, the relay device 15 transmits a connection request forthe relay device 25 to the server device 35 (step S111). As illustratedin FIG. 3, this connection request is an “INVITE request message” of theSIP, and includes TCP connection information in a body portion followinga blank line. In the illustrated example, the connection requestincludes an IP address (200.1.1.1) and a TCP port number (6109) or thelike of the relay device 15 (transmitter). The server device 35 relaysthe connection request to the relay device 25 (step S112). The relaydevice 25, which has received the connection request, transmits aresponse to the server device 35 to permit a connection (step S113). Asillustrated in FIG. 3, this response is “200 OK response message” of theSIP, and includes TCP connection information in a body portion followinga blank line. In the illustrated example, the response includes an IPaddress (200.2.2.2) and a TCP port number (7109) or the like of therelay device 25 (transmitter). The server device 35 relays the responseto the relay device 15 (step S114). As described above, the relay device15 and the relay device 25 use the INVITE request and the OK response ofthe SIP to exchange TCP connection information, and carry out anegotiation for establishing a TCP connection. When the relay device 15receives the response, the relay device 15 transmits a TCP connectionrequest to the relay device 25 (step S115). Accordingly, a TCPconnection is established between the relay device 15 and the relaydevice 25.

The above-described processing is executed by a network administrator ofthe LAN 10 or the LAN 20, for example. That is, in order to prepare suchthat a connection with another LAN can be carried out dynamically, arelay device in a network is logged into the server device 35 by thenetwork administrator as illustrated at step S107. Accordingly, apreparation is completed for receiving a TCP connection request fromanother relay device. When the network administrator wishes to carry outa connection with another LAN, the network administrator accesses theserver device 35, and acquires status of a destination relay device.When the network administrator learns that the destination relay deviceis under a logged-in status, the network administrator transmits aconnection request to the server device 35.

That is, in the present preferred embodiment, the relay device 15transmits an INVITE request of the SIP via the server device 35 toestablish a TCP connection (media session) for relaying. In other words,since a relay channel as a media session is generated by a call controlprotocol, a relay communication channel can be established dynamically.

When a TCP connection is established between the relay device 15 and therelay device 25 as described above, the relay device 15 and the relaydevice 25 hold the established TCP connection. When the relay device 15receives data from the communication device 11, 12 or the like to betransmitted to the communication device 21, 22 or the like, the relaydevice 15 relays the received data to the relay device 25 (step S116).The relay device 25 further relays the relayed data to the communicationdevice 21, 22 or the like. Data transmitted from the communicationdevice 21, 22 or the like is also relayed to the communication device11, 12, or the like via the relay device 25 and the relay device 15(step S117).

Further, in order to carry out communication between the LAN 10 and theLAN 20 via the WAN 30, each of the relay devices 15 and 25 manages alist of device names of communication devices connected to each of theLANs 10 and 20, respectively. When a TCP connection is establishedbetween the relay device 15 and the relay device 25, the relay device 15and the relay device 25 exchange the list. When carrying outcommunication between a communication device connected to the LAN 10 anda communication device connected to the LAN 20, a device name of a relaydevice of a LAN of a transmission destination and a device name of acommunication device of the transmission destination are designated.That is, since a unique device name (registered in the relay devicedatabase 351) is assigned to a relay device using such a communicationsystem, by designating both the device name of the relay device and thedevice name of the communication device, the communication device of thetransmission destination can be identified uniquely. For example, a namesuch as “communication device name@relay device name” may be used. Eachrelay device knows an association between a communication device nameand an IP address for a communication device connected to the LAN towhich the relay device itself is also connected. Therefore, the relaydevice can relay data to a communication device designated by“communication device name@relay device name”.

When transmission and reception of data between the communicationdevices end and the connection established between the relay device 15and the relay device 25 becomes unnecessary, the network administratordisconnects the connection. First, the relay device 15 transmits adisconnection request for the relay device 25 to the server device 35(step S118). The server device 35 relays the disconnection request tothe relay device 25 (step S119). A response from the relay device 25 istransmitted to the server device 35 (step S120), and relayed to therelay device 15 (step S121).

FIG. 4 is a flowchart focusing on a processing carried out by the relaydevice 15 in the processing described with reference to FIG. 3. First,the relay device 15 transmits a starting request for starting a relayconnection to the server device 35 (step S201). The relay device 25 isdesignated as the relay destination in the starting request. When a factthat the relay device 25 of the relay destination is logged out becomesapparent from the response from the server device 35 (step S202: NO),the relay device 15 stands by until receiving a login notification (stepS203). When receiving a login notification (step S203: YES), the processproceeds onto step S204.

When a determination is made at step S202 that the relay device 25 ofthe relay destination is logged in, the relay device 15 generates TCPconnection information (step S204), and transmits a connection request(step S205). Then, the relay device 15 stands by for a response from therelay device 25 (step S206). When receiving a response (step S206: YES),the relay device 15 analyzes the TCP connection information included inthe received response (step S207). That is, the relay device 15 acquiresport number information or the like included in the response transmittedfrom the relay device 25. Then, the relay device 15 carries out a TCPconnection with the relay device 25 (step S208).

Under a state in which a TCP connection is established and held, whenthe communication device 11 or the like transmits relay data (step S209:YES), the relay device 15 relays the data to the communication device 21or the like (step S210). When the relay device 15 receives relay data tobe relayed to the communication device 11 or the like (step S211: YES),the relay device 15 relays the data to the communication device 11 orthe like of the relay destination (step S212). When the relay device 15receives a disconnection request from the relay device 25 (step S213:YES), the relay device 15 transmits a response (step S214) anddisconnects the TCP connection (step S215).

When a disconnection request is carried out by a network administratorof the LAN 10 (step S216: YES), the relay device 15 transmits adisconnection request to the relay device 25 (step S217). When the relaydevice 15 receives a response (step S218: YES), the relay device 15disconnects the TCP connection (step S219).

FIG. 5 is a flowchart focusing on a processing carried out by the relaydevice 25 in the processing described with reference to FIG. 3. First, anetwork administrator of the LAN 20 determines whether or not to preparefor establishing a connection with another LAN. When preparing forestablishing a connection with another LAN, an operation is performedfor validating a relay function of the relay device 25. When the relayfunction is validated (step S301: YES), the relay device 25 transmits alogin command to the server device 35 (step S302). Then, the relaydevice 25 receives a response from the server device 35 (step S303), anda processing for validating the relay function is completed.

As described above, when the relay device 25 of the relay destination isunder a logged-in status, that is, when the relay device 25 is capableof accepting a connection, the relay device 15 dynamically establishes aTCP connection with the relay device 25. Accordingly, data can betransmitted and received via the WAN 30 such as the Internet. Forexample, in case of a headquarter office and a branch office of acompany where traffic constantly generates stably, LANs may be connectedfixedly by using the conventionally used VPN. Meanwhile, when carryingout data communication by establishing connections with differentnetworks via the WAN 30 under any timing, the communication system ofthe present preferred embodiment can be used.

According to the communication system of the present preferredembodiment, when a connection environment (an IP address and/or a portnumber) of the relay device of the other party is changed, the relaydevice carries out a negotiation first (steps S111 through S114).Therefore, a connection for relaying data can be established reliably.

The relay devices 15 and 25 of the present preferred embodiment canestablish a TCP connection with a plurality of relay devices. Forexample, as illustrated in FIG. 6, the relay device 15 can establish aseparate TCP connection with each of three relay devices 25, 45 and 55.A method for establishing a TCP connection with the relay devices 45 and55 is the same as the processing carried out with respect to the relaydevice 25. Each of the relay devices 15 and 25 or the like establishes aTCP connection with a plurality of relay devices, and relays data to theplurality of the relay devices. In an example illustrated in FIG. 6, therelay device 15 relays data transmitted from a certain communicationdevice to the relay device 45, and relays data transmitted from anothercommunication device to the relay device 55.

A method for disconnecting the TCP connection established with each ofthe relay devices 45 and 55 is also the same as the processing carriedout with respect to the relay device 25. That is, a TCP connection canbe established independently with each of the plurality of the relaydevices 25, 45, and 55, and the TCP connection can be disconnectedindependently.

Conventionally, in the VPN or the like, when a plurality of LANs areconnected, the plurality of the LANs are connected as one VPN.Meanwhile, according to the communication system of the presentpreferred embodiment, a connection is established independently withjust a relay device to which a communication is to be carried out, and aconnection established with a relay device to which a communication isno longer necessary to be carried out can be disconnected independently.Therefore, efficient communication system can be constituted withoutconsuming unnecessary resources.

Second Preferred Embodiment

Next, a second preferred embodiment of the present invention will bedescribed. In the first preferred embodiment, a connection isestablished dynamically between relay devices according to aninstruction from a network administrator. In the second preferredembodiment, a connection is established between relay devices accordingto a designation from a communication device. Specifically, in the firstpreferred embodiment, by predicting a status of communication to begenerated between the communication devices, a network administratorcarries out an operation for establishing or disconnecting a connectionbetween the relay devices. Meanwhile, in the second preferredembodiment, a control is carried out to establish or disconnect aconnection between the relay devices more dynamically according to acommunication processing that generates from a communication device. Inthe following description, a description that is the same as the firstpreferred embodiment will be omitted.

FIG. 7 illustrates an example of a configuration of a communicationsystem according to the second preferred embodiment of the presentinvention. In FIG. 7, a LAN 40 is added to the system configurationillustrated in FIG. 1. The basic system configuration illustrated inFIG. 7 is the same as the first preferred embodiment. A communicationdevice 41 and a relay device 45 are connected to the LAN 40, and the LAN40 is connected to a WAN 30 via a gateway 46.

The relay devices 15, 25, and 45 include relay connection databases 151,251, and 451, respectively. FIG. 8 illustrates an example ofregistration of the relay connection database 151 stored in the relaydevice 15.

The relay connection database 151 is a database for managing a TCPconnection currently established by the relay device 15. A “client”field stores a setting of a device name of a communication device thathas requested a connection to be established with a relay device(hereinafter referred to as a “requesting communication device”). Forexample, “ClientX” and “ClientY” in FIG. 8 are a device name assigned tothe communication device 11, 21 or the like. “Relay destination URL” and“IP address” fields store a setting of a URL and an IP address of arelay device of a relay destination, respectively. “Connection number”field stores a setting of a port number of a generated TCP connection.“Generated time” field stores a setting of time when a TCP connectionhas been established.

In the present preferred embodiment, according to a designation from therequesting communication device, a TCP connection is established with arelay device of a communication destination. However, a device that canuse such a TCP connection for carrying out communication is only therequesting communication device. That is, as illustrated in FIG. 8,one-to-one relation is established for a communication device and a TCPconnection.

The contents of the relay connection databases 251 and 451 are the sameas the relay connection databases 151 illustrated in FIG. 8. The relayconnection databases 251 and 451 include registration of a status of aTCP connection currently established by the relay devices 25 and 45,respectively.

Next, with reference to FIG. 9 through FIG. 11, a description will bemade of a flow of a communication processing carried out in theabove-described communication system. FIG. 9 illustrates a flow of theprocessing of the entire communication system including thecommunication devices 11 and 12, the relay devices 15 and 25, and theserver device 35. Further, in the following description, a descriptionwill be made of an example in which the SIP is used for thecommunication between the relay devices 15 and the server device 35 andbetween the server device 35 and the relay device 25. However, otherprotocols may also be used.

In the same manner as in the first preferred embodiment, each of therelay devices 15 and 25 transmits a “REGISTER request message” of theSIP to the server device 35 at initialization or periodically to notifyposition information (for example, an IP address and/or a port number)of the relay device to the server device 35. In FIG. 9, under an initialstate, the relay device 15 and the relay device 25 are logged in theserver device 35. First, the relay device 25 notifies logout statusinformation to the server device 35 (step S401), and the server device35 responds to the notified logout status information (step S402). Theserver device 35 executes a logout processing of the relay device 25,and updates the “status” field of the relay device 25 in the relaydevice database 351 to “logout”.

Under this state, the communication device 11 transmits a relaytransmission request of data and a status confirmation request, whichdesignate the relay device 25, to the relay device 15 (step S403). Whenthe relay device 15 carries out a notification request of the statusinformation to the server device 35 (step S404), the server device 35carries out a response (step S405) and notifies the relay device 15 thatthe relay device 25 is logged out (step S406). The relay device 15responds to the notification (step S407). Furthermore, the relay device15 notifies the communication device 11 that the relay device 25 is alogged out (step S408). Accordingly, the communication device 11 standsby until the relay device 25 logs in.

Next, the relay device 25 notifies login status information to theserver device 35 (step S409), and the server device 35 responds to thenotified login status information (step S410). The server device 35executes a login processing of the relay device 25, and updates the“status” field of the relay device database 351 to “login”. The serverdevice 35 also notifies the relay device 15 that the relay device 25 haslogged in (step S411). The relay device 15 responds to the notification(step S412). Furthermore, the relay device 15 notifies the communicationdevice 11 that the relay device 25 has logged in (step S413).

When the communication device 11, which has been on standby, receivesthe notification carried out at step S413, the communication device 11carries out a relay transmission request again designating the relaydevice 25 (step S414). Further, in the second preferred embodiment, thecommunication device 11 is provided to standby until receiving anotification that the relay device 25 has logged in. However, as anotherpreferred embodiment, without standing by for such a notification, thecommunication device 11 may periodically transmit a relay transmissionrequest to the relay device 15.

When the relay device 15 receives the relay transmission request fromthe communication device 11, the relay device 15 transmits a connectionrequest for the relay device 25 to the server device 35 (step S415). Asdescribed in the first preferred embodiment, the connection request isan “INVITE request message” of the SIP, and includes TCP connectioninformation. The server device 35 relays the connection request to therelay device 25 (step S416). When receiving the connection request, therelay device 25 transmits a response to the server device 25 to permit aconnection (step S417). As described in the first preferred embodiment,the response is “200 OK response message” of the SIP, and includes TCPconnection information. The server device 35 relays the response to therelay device 15 (step S418). The relay device 15 and the relay device 25exchange the TCP connection information as described above. Whenreceiving the response, the relay device 15 transmits a TCP connectionrequest to the relay device 25 (step S419). Accordingly, a TCPconnection is established between the relay device 15 and the relaydevice 25.

The above-described processing is carried out dynamically, for example,when a communication processing from the communication device 11 to thecommunication device 21 generates. That is, the relay devices are notconnected fixedly, but a connection is carried out when trafficgenerates. However, in order to prepare for a connection request from arelay device of another end, a network administrator is required to beprepared by logging a relay device within a network into the serverdevice 35 as illustrated at step S409.

In the present preferred embodiment, the relay device 15 receives aninstruction from the communication device 11, and the relay device 15transmits an INVITE request of the SIP via the server device 35 toestablish a relay TCP connection (media session). That is, at a point oftime when a connection request from a communication device generates,since a relay channel as a media session is generated by a call controlprotocol, a relay communication channel can be established dynamically.

When a TCP connection is established between the relay device 15 and therelay device 25 as described above, the relay device 15 and the relaydevice 25 hold the established TCP connection. When the communicationdevice 11 transmits data addressed to the communication device 21, 22 orthe like (step S421), the relay device 15 relays the data to the relaydevice 25 (step S422). The relay device 25 further relays the relayeddata to the communication device 21, 22 or the like. The datatransmitted from the communication device 21, 22 or the like is relayedto the relay device 15 via the relay device 25 in the same manner (stepS423) and relayed further to the communication device 11 via the relaydevice 15 (step S424).

When a relay transmission using the relay device 25 is completed, thecommunication device 11 transmits a disconnection instruction to therelay device 15 (step S425). The relay device 15 transmits adisconnection request for the relay device 25 to the server device 35(step S426). The server device 35 relays the received disconnectionrequest to the relay device 25 (step S427). A response from the relaydevice 25 is transmitted to the server device 35 (step S428) and relayedto the relay device 15 (step S429). Accordingly, the TCP connectionbetween the relay device 15 and the relay device 25 is disconnected, andthe relay device 15 notifies the communication device 11 that the TCPconnection has been disconnected (step S430).

Under such a state, when another communication device 12 carries out arelay transmission request designating the relay device 25 again (stepS431), a connection request is transmitted from the relay device 15 tothe server device 35 (step S432). At this point of time, when the relaydevice 25 is under a logged-out status, the server device 35 notifiesthe logged-out status (step S433), and the communication device 12 isnotified that the relay device 25 is under a logged-out status. In thesame manner as the processing carried out by the communication device11, when the relay device 25 logs in, the communication device 12carries out a relay transmission request to the relay device 25 again.Accordingly, a TCP connection is established between the relay device 15and the relay device 25.

FIG. 10 is a flowchart focusing on a processing carried out by the relaydevice 15 in the processing described with reference to FIG. 9. First,the relay device 15 confirms whether or not a relay instruction from thecommunication device 11, 12 or the like is generated (step S501). When arelay instruction is generated (step S501: YES), the relay device 15confirms whether or not a status monitoring instruction is generated(step S502). When the status monitoring instruction is generated, therelay device 15 checks whether or not the relay device of the relaydestination is logged in (step S503). When the relay device 15 confirmsthat the relay device of the relay destination is logged out (step S503:NO), the relay device 15 stands by until receiving a login notification(step S504).

When receiving a notification from the server device 35 indicating thatthe relay device of the relay destination has logged in (step S504:YES), the relay device 15 notifies the requesting communication devicethat the relay destination has logged in (step S505). Then, the relaydevice 15 stands by until receiving a relay instruction from thecommunication device 11, 12 or the like again (step S506). When therelay device 15 receives a relay instruction, the process proceeds ontostep S508. When a determination is made at step S503 that the relaydevice of the relay destination is logged in, the process proceedsimmediately onto step S508. When a status monitoring instruction is notincluded in the request from the communication device (step S502: NO),if the relay destination is logged in (step S507: YES), the processproceeds onto step S508. If the relay destination is not logged in, theprocess returns to step S502 and the processing is repeated. That is,when a status monitoring instruction has been received from a requestingcommunication device, the relay device 15 monitors the relay device ofthe relay destination until the relay device of the relay destinationlogs in. When receiving a notification that the relay device of therelay destination has logged in, the relay device 15 notifies suchinformation also to the requesting communication device.

Next, the relay device 15 generates TCP connection information (stepS508) and transmits a connection request (step S509). The relay device15 stands by until receiving a response from the relay device 25 (stepS510). When not receiving a response (step S510: NO), the relay device15 carries out an error notification to the requesting communicationdevice (step S511). The process returns to step S501 and the processingis repeated. When receiving a response (step S510: YES), the relaydevice 15 analyzes the TCP connection information included in thereceived response (step S512). That is, the relay device 15 acquiresport number information or the like included in the response transmittedfrom the relay device 25. Then, the relay device 15 carries out a TCPconnection to the relay device 25 (step S513). The relay device 15notifies the requesting communication device that a connection has beenestablished with the relay destination (step S514). Then, the relaydevice 15 registers information of the newly established TCP connectionin the relay connection database 151 (step S515).

Under a state in which the TCP connection is established and held, inthe flowchart of FIG. 11, when relay data is transmitted from thecommunication device 11 or the like (step S516: YES), the relay device15 relays the data to the communication device 21 or the like (stepS517). When receiving relay data to be relayed to the communicationdevice 11 or the like (step S518: YES), the relay device 15 relays thedata to the communication device 11 or the like of the relay destination(step S519). When receiving a disconnection request from the relaydevice 25 (step S520: YES), the relay device 15 transmits a response(step S521) and disconnects the TCP connection (step S522). When adisconnection request is carried out by the requesting communicationdevice (step S523: YES), the relay device 15 transmits a disconnectionrequest to the relay device 25 of the relay destination (step S524).When receiving a response (step S525: YES), the relay device 15disconnects the TCP connection (step S526). Then, the relay device 15notifies the requesting communication device that a connection with therelay destination has been disconnected (step S527), and the informationof the TCP connection registered in the relay connection database 151 isdeleted (step S528).

As described above, according to the second preferred embodiment, when arelay instruction from a communication device generates, a TCPconnection is established dynamically with the relay device of the relaydestination, and communication can be carried out between privatenetworks. Unlike the conventional VPN or the like that is fixedly set,when a request of communication generates, a connection necessary forsuch communication is established. Therefore, resources can be utilizedefficiently.

According to the second preferred embodiment, when a communicationconnection is established according to a request from a certaincommunication device, only such a requesting communication device ispermitted to use the established communication connection. For example,when a request of communication generates from the communication device11 to the communication device 21, a TCP connection is establishedbetween the relay device 15 and the relay device 25 according to therequest. The established TCP connection is used only for thecommunication between the communication device 11 and the communicationdevice 21. In other words, the relay device 15 and the relay device 25use the TCP connection just for relaying the data transmitted andreceived between the communication device 11 and the communicationdevice 21. Therefore, a connection established according to a requestfrom a certain communication device is used exclusively by such acommunication device.

In the example illustrated in FIG. 7, under a state in which aconnection is established between the relay device 15 and the relaydevice 25 according to a request from the communication device 11,suppose that a connection is established further between the relaydevice 15 and the relay device 45 according to a request from thecommunication device 12. In this case, a description has been made withreference to FIG. 6 that an individual data relay connection isestablished. Therefore, the communication device 11 and thecommunication device 12 use different TCP connections for relaying data.Meanwhile, under a state in which the communication device 11 iscarrying out a relay transmission designating the relay device 25, whenthe communication device 12 also carries out a relay instructiondesignating the same relay device 25, since the already established TCPconnection is used exclusively by the communication device 11, aseparate communication connection is established.

Third Preferred Embodiment

Next, a description will be made of a third preferred embodiment of thepresent invention. Also in the third preferred embodiment, in the samemanner as in the second preferred embodiment, a connection is carriedout between relay devices according to a request from a communicationdevice. However, in the third preferred embodiment, when a connectionhas already been established between the relay devices, anothercommunication device shares the already established connection. Thisfeature differs from the second preferred embodiment. In the following,a description will be made primarily of aspects that are different fromthe second preferred embodiment.

FIG. 12 illustrates an example of registration of the relay connectiondatabase 151 stored in the relay device 15. As illustrated in FIG. 12, aplurality of clients (communication devices) are associated with one TCPconnection. In the example illustrated in FIG. 12, a relay transmissionrequest has generated from two communication devices (ClientX andClientY), and both of the communication devices designate the same relaydevice (Relayserver1@sample.net). Therefore, the two communicationdevices are sharing a TCP connection with a connection number 49583.

The contents of the relay connection databases 251 and 451 stored in therelay devices 25 and 45, respectively, are the same as the relayconnection database 151 illustrated in FIG. 12. The relay connectiondatabases 251 and 451 store a status of a TCP connection currentlyestablished by the relay devices 25 and 45, respectively.

Next, with reference to FIG. 13 through FIG. 15, a description will bemade of a flow of a communication processing carried out in theabove-described communication system. FIG. 13 illustrates a flow of aprocessing of the entire communication system including thecommunication devices 11 and 12, the relay devices 15 and 25, and theserver device 35. Further, in the following description, a descriptionwill be made of an example in which the SIP is used for thecommunication between the relay device 15 and the server device 35, andbetween the server device 35 and the relay device 25. However, otherprotocols may also be used.

The processing from step S701 to step S724 in FIG. 13 corresponds to theprocessing from step S401 to step S424 in FIG. 9. Since the processingis the same, a description will be omitted. That is, the communicationdevice 11 generates a relay transmission request designating the relaydevice 25, and a TCP connection is established between the relay device15 and the relay device 25. Then, data is transmitted and receivedbetween the communication device 11 and the communication device 21 orthe like.

Under such a state, a different communication device 12 carries out arelay transmission request designating the relay device 25 again (stepS725). Since the TCP connection is currently being established betweenthe relay device 15 and the relay device 25, the relay device 15notifies the communication device 12 that a relay transmission of thedata can be carried out (step S726). Accordingly, when data istransmitted from the communication device 12 (step S727), the relaydevice 15 uses the already established TCP connection to relay the datatransmitted from the communication device 12 to the relay device 25(step S728). The relay device 25 also uses the already established TCPconnection for relaying the data, which has been transmitted from thecommunication device 21 or the like to the communication device 12, tothe relay device 15 (step S729). The relay device 15 further transmitsthe received data to the communication device 12 (step S730).

As described above, in the third preferred embodiment, a TCP connectionis already established between the relay devices, and when the alreadyestablished TCP connection can be used for a newly generated relaytransmission request, a new TCP connection is not established. Aplurality of communication terminals share the same TCP connection.Accordingly, load required for establishing a TCP connection can bereduced, and communication between different networks can be carried outunder higher speed. In addition, resources of the relay device can besaved. For example, when carrying out communication via the Internetbetween a plurality of communication terminals connected to the LAN 10and a plurality of communication terminals connected to the LAN 20, anincrease in the processing speed and saving of the resources arepossible and efficient. The above-described third preferred embodimentis also efficient when data volume to be relayed from one communicationdevice is small.

Under a state in which the communication device 11 and the communicationdevice 12 are sharing the TCP connection and carrying out datacommunication as described above, the communication device 11 transmitsa disconnection instruction to the relay device 15 (step S731). Sincethe communication device 12 is using the same TCP connection, the relaydevice 15 does not carry out a processing in particular with respect tothe disconnection instruction of step S731, and just carries out aresponse (step S732).

Then, when the relay transmission using the relay device 25 ends, thecommunication device 12 transmits a disconnection instruction to therelay device 15 (step S733). Since the relay device 15 has alreadyreceived the disconnection instruction from the communication device 11,the relay device 15 can determine that the shared TCP connection hasbecome unnecessary. Therefore, the relay device 15 transmits adisconnection request for the relay device 25 to the server device 35(step S734). The server device 35 relays the disconnection request tothe relay device 25 (step S735). The relay device 25 transmits aresponse to the server device 35 (step S736). Then, the server device 35relays the response to the relay device 15 (step S737). Accordingly, theTCP connection between the relay device 15 and the relay device 25 isdisconnected, and the relay device 15 notifies the communication device11 that the TCP connection has been disconnected (step S738).

FIG. 14 is a flowchart focusing on a processing carried out by the relaydevice 15 in the processing described with reference to FIG. 13. First,the relay device 15 confirms whether or not a relay instruction from thecommunication device 11 or 12 or the like has generated (step S801).When a relay instruction has generated (step S801: YES), the relaydevice 15 confirms whether or not a TCP connection is alreadyestablished with a relay device of a relay destination (step S802). Thatis, the relay device 15 confirms whether or not the same relay devicehas already been designated by another communication device and acommunication is being carried out with such a relay device.

When a connection is already established with the already instructedrelay destination (step S802: YES), the relay device 15 notifies thatdata can be relayed to a requesting communication device (step S815).Then, the relay device 15 updates the information of the relayconnection database 151 (step S816). That is, since informationregarding the TCP connection is already registered in the relayconnection database 151, the relay device 15 just adds a client(communication device). In the example illustrated in FIG. 12, forexample, when ClientX and a TCP connection with a connection number49583 are associated and stored, the relay device 15 carries out aprocessing for adding ClientY to the “client” field of such record.

When a connection is yet to be established with the designated relaydestination at step S802, the relay device 15 executes the processingfrom step S803 to step S814. This processing corresponds to theprocessing from step S502 to step S513 in FIG. 10. Since the processingis the same, a description will be omitted.

Next, a description will be made with reference to FIG. 15. Under astate in which a TCP connection is established and held, when relay datais transmitted from the communication device 11 or the like (step S817:YES), the data is relayed to the communication device 21 or the like(step S818). When receiving relay data to be relayed to thecommunication device 11 or the like (step S819: YES), the relay device15 relays the data to the communication device 11 or the like of therelay destination (step S820). When receiving a disconnection requestfrom the relay device 25 of the relay destination (step S821: YES), therelay device 15 transmits a response (step S822) and disconnects the TCPconnection (step S823). As described above, when the TCP connection isdisconnected from the relay device 25 of the relay destination, therelay device 15 notifies the communication device to which relaytransmission is being carried out that the connection with the relaydestination has been disconnected to (step S824). When the disconnectedTCP connection had been shared among a plurality of communicationdevices, the disconnection is notified to all of the communicationdevices (steps S824 and S825).

When a disconnection request is carried out by the requestingcommunication device (step S826: YES), the relay device 15 checkswhether or not the TCP connection instructed to be disconnected is beingshared with another communication device (step S827). When such a TCPconnection is being shared with another communication device (step S827:YES), without disconnecting the TCP connection, a formal disconnectionnotification is carried out to the communication device that has carriedout the disconnection instruction (step S828). Then, the relay device 15updates the information stored in the relay connection database 151(step S829). That is, the relay device 15 deletes information of theclient (communication device) that has requested the disconnectioninstruction from the information of the shared TCP connection. The relaydevice 15 manages a currently established TCP connection andcommunication devices sharing the TCP connection at all times asdescribed above.

When the TCP connection instructed to be disconnected is not shared withanother communication device (step S827: NO), the relay device 15transmits a disconnection request to the relay device 25 (step S830).When receiving a response (step S831: YES), the TCP connection isdisconnected (step S832). Then, the relay device 15 notifies therequesting communication device that the connection with the relaydestination has been disconnected (step S833) and deletes theinformation of the TCP connection registered in the relay connectiondatabase 151 (step S834).

As described above, according to the third preferred embodiment, when arelay instruction from a communication device generates, a TCPconnection is dynamically established with a relay device of a relaydestination, and communication can be carried out between privatenetworks. Furthermore, when a relay instruction generates, if a TCPconnection is already established with the relay destination, thealready established TCP connection is shared. Accordingly, speed of theprocessing is increased, and resources can be utilized efficiently.

While the present invention has been described with respect to preferredembodiments thereof, it will be apparent to those skilled in the artthat the disclosed invention may be modified in numerous ways and mayassume many embodiments other than those specifically set out anddescribed above. Accordingly, the appended claims are intended to coverall modifications of the present invention that fall within the truespirit and scope of the present invention.

What is claimed is:
 1. A first relay device capable of communicatingwith a first communication device connected to a first local areanetwork (LAN) and a second communication device connected to the firstLAN, a server device connected to a wide area network (WAN), a secondrelay device connected to the WAN and a second LAN, the first relaydevice comprising: a holding unit for establishing and holding atransmission control protocol (TCP) relay connection with the secondrelay device by inquiring of the server device as to a connection statusof the second relay device and transmitting a request for connecting thesecond relay device to the server device when being notified by theserver device that the second relay device has logged into the serverdevice; a storing unit for storing connection information in which thesecond relay device, the first communication device and the TCP relayconnection held in the holding unit are associated with each other, andfor storing connection information in which the second relay device, thesecond communication device and the TCP relay connection held in theholding unit are associated with each other; a determining unit fordetermining whether a TCP relay connection associated at least with thesecond relay device has been held or not based on data, which isreceived from the first communication device, designating the secondrelay device, and the connection information stored in the storing unit;a relay connection controlling unit, when the first relay devicereceives data designating the second relay device from the firstcommunication device, for transmitting the data to the second relaydevice via the TCP relay connection held in the holding unit where thedetermining unit determines that a TCP relay connection associated atleast with the second relay device has been held, and, when the firstrelay device receives data designating the second relay device from thesecond communication device, for transmitting the data to the secondrelay device using the same TCP relay connection used by the firstcommunication device to transmit data to the second relay device, andfor transmitting the data to the second relay device via a new TCP relayconnection which the holding unit is directed to dynamically establishwith the second relay device and to hold where the determining unitdetermines that a relay connection associated at least with the secondrelay device is yet to be held; and an updating unit for updatingconnection information stored in the storing unit.
 2. The first relaydevice according to claim 1, wherein the holding unit, when receiving anotification from the first communication device to terminate a relaytransmission to the second relay device, refers to the storing unit, andwhere the other communication device is carrying out relay transmissionto the second relay device, the holding unit holds the TCP relayconnection with the second relay device, and where no one of the othercommunication devices is carrying out relay transmission to the secondrelay device, the holding unit disconnects the TCP relay connection withthe second relay device.